Fuel cells are capable or converting electrochemical energy into electrical energy, and as such perform the same function as batteries. However, unlike batteries that run down and require recharging, fuel cells produce electricity as long as fuel is supplied to the cell. Fuel cells are also highly efficient devices. For example, fuel cells may convert fuels to useful energy at an efficiency as high as 60 percent, whereas internal-combustion engines are limited to efficiencies of near 40 percent or less. In addition, fuel cells do not necessarily depend on fossil fuels, do not emit noxious gases such as nitrogen dioxide and produce virtually no noise during operation. Fuel cells are therefore attractive sources of electrical power, and are being developed for use in power stations, car engines, portable electronic devices, etc.
The simplest of fuel cells consist of two electrodes (an anode and a cathode) sandwiched around an electrolyte. Hydrogen fuel is fed to the anode and oxygen (or air) is fed to the cathode. In the presence of a catalyst such as platinum, the hydrogen atom splits into a proton and an electron at the anode. The proton and electron then proceed along separate paths to the cathode; while the proton reaches the cathode via the electrolyte the electron creates a separate current through an electrical circuit. The proton and electron reunite at the cathode and react with oxygen to produce water. Overall, the electrochemical reactions involved are:
At the anode: 2H2xe2x86x924H++4exe2x88x92
At the cathode: O2+4exe2x88x92+4H+xe2x86x922H2O
Overall: 2H2+O2xe2x86x922H2O
In order to maximize the contact area available between the hydrogen fuel, the oxygen, the electrode, and the electrolyte, and in order to minimize the distance that the protons need to travel between the electrodes, the electrodes and electrolyte are usually made to be flat and thin. In addition, the structure of the electrodes is usually porous.
The voltage produced between the anode and cathode of a fuel cell is typically on the order of about 0.7 V. As a consequence, in order to produce a practical voltage (e.g., between about 10 and 100 V) many fuel cells need to be connected in series. Such a collection of fuel cells is referred to as a fuel cell xe2x80x9cstackxe2x80x9d. The preferred method of connecting neighboring fuel cells in a stack involves separating them with xe2x80x9cbipolar platesxe2x80x9d. The bipolar plates must provide a good electrical connection between the anode and cathode of neighboring fuel cells and provide a means of feeding hydrogen to the anode of one fuel cell, oxygen to the cathode of its neighbor, all the while preventing the hydrogen and oxygen from mixing.
A bipolar plate is typically constructed by creating flow channels on two opposing faces of a conductive substrate (e.g., graphite or a metal). The flow channels provide a flow system for feeding hydrogen or oxygen to the electrodes, and the plateaus defined between the flow channels provide an electrical contact between the neighboring electrodes.
Existing fuel cells utilize bipolar plates that are machined out of solid graphite or metals, molded or pressed from composite graphitic materials, or formed out of the fuel cell material itself. These methods can be very expensive, have material compatibility limitations, and do not typically offer the stack designer a great amount of design flexibility, especially with respect to feature size and complexity. The limitations of current bipolar plate design and construction may become severe restrictions in the development of future fuel cells, particularly in the development of small scale portable fuel cells that may require very thin, substantially planar plates with narrow flow features. For example, graphitic plates are both brittle and porous and therefore break when machined and/or become permeable to hydrogen and oxygen fluids below a certain thickness.
In addition, in order to produce fuel cells that operate under stable conditions, there is a need in the art for the development of a variety of sensors and devices that would allow a variety of fuel cell parameters such as fuel flow rates, electric current load, gas and liquid pressures, fuel or cell temperature, etc. to be both monitored and controlled. Ideally, these sensors and devices could be incorporated within the relatively bulky bipolar plates; however, current bipolar plate construction and design methods do not provide the tools necessary for such developments and fine detail.
Accordingly, it would be desirable to provide bipolar and end plates and methods of making bipolar and end plates for fuel cells that overcome these limitations.
The present invention addresses and solves the above-mentioned problems by providing bipolar plates for fuel cell stacks that include a semi-conductive or a conductive body having a first face adapted to contact an anode of a first fuel cell and a second face adapted to contact a cathode of a second fuel cell. The first face comprising a flow channel adapted to confine fuel fluids, and the second face comprising a flow channel adapted to confine oxidizing fluids.
A method of making the bipolar plate of the present invention comprises the steps of providing a semi-conductive or conductive substrate having a first and a second face and etching a flow channel from both the first face and the second face.